1. Field of the Invention:
This invention relates to a manufacturing method and apparatus and more particularly to an apparatus for forming dissimilar polymer compositions into composite articles such as composite contact lenses.
2. Description of the Prior Art:
Composite contact lenses known as hybrid lenses have a hard center and a soft periphery. A central segment is formed of a rigid or hard plastic material and is bonded to a peripheral segment formed of a softer hydrophilic polymer. A virtue of such lenses is that the hard center, being resistant to distortion, has good optical properties while the soft periphery permits the lens to fit the eye more comfortably than would a completely hard lens.
A problem with hybrid lenses is the difficulty of manufacture and the difficulty of obtaining a satisfactory bond between the dissimilar polymers of the central and peripheral segments. For example, the patent to Graham, U.S. Pat. No. 4,166,255 discloses hybrid lenses which are manufactured by preforming the hard central segment, placing it in a mold, pouring the monomer for the soft segment over it and curing the latter. The Graham patent also discloses a method in which a preformed hard segment is encased on all sides in the soft polymer, evidently to obtain a better bond than would be achieved by the first-mentioned method. A lens of this kind is difficult to manufacture. Also stresses resulting from the wide area of contact of the two different polymers can distort the shape of the lens. Another method suggested by the patent, evidently also to obtain a better bond, is to diffuse the hard polymer into a central section composed of the soft polymer. In this method controlling the physical and optical properties of the polymer blend at the center of the lens is a problem.
The patent to Deichert et al U.S. Pat. No. 4,208,362 discloses a method for making composite lenses in which the bonding of polymers is improved by polymerizing a first monomer while inhibiting polymerization at the surface which is to be bonded to a second monomer. The second monomer is then contacted with that surface and polymerized. At the junction a random copolymer of the two monomers forms. This provides a transition section which bonds the two dissimilar polymers. Although the Deichert method is useful, it too has disadvantages. For instance, when the first liquid monomer is placed in a mold, the surface activity of the mold material and of the monomer may cause the monomer to creep beyond the desired level on the mold surface. When that monomer is then polymerized a polymer film is formed in the area that is intended for the second polymer. It is thus difficult to control the dimensions of the first polymer in the mold.
The patent to Kamath, U.S. Pat. No. 3,619,044 discloses a method for forming hybrid lenses in which a coating of polyacrylic acid is grafted onto a preformed rod of poly(4-methylpentene-1). Lens buttons are sliced from the coated rod. The buttons are then formed into lenses and are hydrated. A drawback of this technique lies in the difficulty of machining the buttons to a desired shape so that when hydrated the lenses will have the correct dimensions.
3. Brief Summary of the Invention
The present invention offers an improved method and apparatus for forming composite articles from dissimilar polymer compositions. In particular the novel method and apparatus produce articles having a good bond between the polymers and provide improved control of the dimensions of the different polymer segments in the composite articles. Although the invention is not limited to the manufacture of such articles, these advantages are especially valuable in the manufacture of composite or hybrid contact lenses.
The method of the invention comprises placing a quantity of a first polymerizable liquid vinyl monomer composition on a surface, exposing only a portion of the monomer to a predetermined pattern of actinic radiation and polymerizing the exposed portion, contacting at least an edge of the polymerized product with a second polymerizable liquid vinyl monomer composition which differs from the first monomer composition and polymerizing the second monomer to create a composite article having segments of dissimilar polymer compositions joined together by a vinyl copolymer bond.
In a preferred embodiment of the invention, the first monomer forms a relatively hard polymer, the second monomer forms a relatively soft polymer and the composite article is a composite contact lens.
In another preferred embodiment, the first monomer composition forms a transparent polymer that is free of solid particles, the second monomer forms a polymer that contains a dispersion of finely divided light-reflective solid particles and both polymers are tinted.
The apparatus of the invention comprises a means for supporting a layer of monomer during polymerization, a source of actinic radiation positioned to expose the surface of the monomer layer, means for confining the exposure of the monomer layer to a predetermined pattern, and means for providing a uniform intensity of the actinic radiation throughout the pattern of exposure, the means for supporting the layer of monomers being substantially non-reflective of the actinic radiation.
In preferred embodiments of the apparatus the radiation source is a source of ultraviolet radiation, the apparatus includes means for collimating the radiation and includes an opaque mask having an aperture of pre-determined pattern for transmitting ultraviolet radiation.